Acid-based fixing solutions are used for the processing of a variety of photographic media, including, for example, photographic film, photographic paper, and X-ray film. These fixing solutions are typically composed of several components. Usually, the primary agent is a chemical constituent which promotes the solubilization of unexposed silver halide found in many film emulsions. The chemical constituent most commonly used for this purpose is thiosulfate ion. Another important component of acid fixing solutions is a pH buffer which typically is an acid. Acetic acid is the most commonly used pH buffer in commercially available photographic fixing solutions today. The presence of an acidic environment helps to neutralize residual developer chemicals from preceding processing steps and thereby reduces the possibility of development continuing in the fixing bath. Typically, commercial fixing solutions also contain a preservative such as sulfite ion.
Prior to about 1940, most thiosulfate fixing baths used sodium thiosulfate as the silver halide solvent. During this time, studies on pH control of the acid fixing bath were conducted with the aim of stopping further development of the photographic film or paper by neutralizing developer alkali carried over from the developing step into the fixing solution. It was found that acids generally would serve this purpose. Organic acids, such as acetic, formic, oxalic, malic, and diglycolic acid, were found to be satisfactory pH buffers and found to be superior to strong inorganic acids. For many reasons, acetic acid became the acid of choice in maintaining the pH of commercial fixing solutions. At present, many major commercial fixing solutions contain acetic acid/acetate ion buffer to maintain the proper pH.
Since about 1940, ammonium thiosulfate has been used as an alternative to sodium thiosulfate as the silver solvent and is today the most common silver halide solvent used in fixing solutions. This is due in large measure to the discovery that the ammonium ion dramatically increases the speed of fixation by promoting the rate of dissolution of silver halide (Alnutt et al., J.SMPE 1943, 41: 300). Because prior studies had already established the acetic acid/acetate ion buffer as suitable for sodium thiosulfate fixing solutions, formulations containing ammonium thiosulfate have also usually included this buffer.
However, an unpleasant and potentially hazardous problem associated with fixing solutions is the characteristic odor which is caused by the presence of certain volatile compounds. The odor from fixing solutions is a major contributor to the unpleasant smell encountered in photo labs and in "one-hour" photo and film processing shops. The odor originates from both the acetic acid component of the pH buffer and the ammonia that is present in rapid fixing baths containing ammonium thiosulfate. In view of the foregoing, a need exists for a fixing composition which retains most or all of the attributes of prior fixing solutions, but which is essentially devoid of objectionable or unpleasant odors.